This invention relates to a control system for maintaining, in a prescribed horizontal orientation, the assembly which supports an earth working tool to an earth working machine. For purposes of explanation, this invention will be described as controlling the blade circle and drawbar assembly for supporting a grader blade to a grading machine.
In view of today's road construction requirements, particularly high speed travel over modern highways, the demand for greater accuracy in preparing road beds for surfacing is substantial. At the same time, the grading operation must be accomplished quickly and efficiently in order to cope with the long distances which today's modern highways are to span. The grading operation performed by today's grading machines must result in quick and efficient operation to yield a highly accurate graded road bed.
In the modern grading machine, the grader blade has many degrees of freedom of movement. For example, the grader blade can be raised or lowered for controlling the grade of the blade, can be raised at one end and/or lowered at the other end for controlling the transverse slope of the blade, can be shifted from side to side to make cuts adjacent to the machine and can be rotated about an axis perpendicular to the blade circle. As long as the blade circle and drawbar assembly of the grading machine is maintained parallel to the line of flight of the machine (i.e., parallel to the cut surface), any rotation about the axis perpendicular to the blade circle will not alter the transverse slope of the blade. However, if the blade circle and drawbar assembly is not parallel to the line of flight of the machine, such rotation will result in a change in the transverse slope of the blade.
One prior art method for trying to compensate for such changes in slope is to mount the slope sensor on the blade circle itself. However, because the sensor is mounted on the blade circle, the slope sensor rotates as the circle rotates. The slope sensor, therefore, does not experience the transverse slope of the blade when projected upon a plane which is perpendicular to the line of flight of the machine. That is, the output from the slope sensor becomes both a function of the transverse slope of the blade and the angle of the fore-aft axis of the machine with respect to a true horizontal axis.
One successful method of compensating for the changes in transverse slope of the grader blade as the blade circle is rotated, is shown in patent applications 408,778 filed Oct. 23, 1973, now abandoned and 548,500 filed Feb. 10, 1975, now abandoned, both of which are assigned to the assignee of the present invention. The device shown in those applications comprises a platform for mounting the slope sensor and a mechanism for providing a correction factor to the slope of the platform dependent upon blade circle rotation. In this manner, the slope sensor is mounted in a fixed relationship with the line of flight of the machine such that it is now influenced by the angle of the fore-aft axis of the machine with respect to a true horizontal axis.
Another alternative, known in the prior art, is to maintain the blade circle and drawbar assembly parallel to the machine frame by the use of an on-off system or a complicated parallelogram linkage proportional control system having a parallelogram linkage connected between the front of the machine and the tow point of the drawbar for controlling a hydraulic cylinder connected between the drawbar and the tow point. On-off systems are incapable of providing both high positioning speeds and a narrow dead band (dead band here is defined as the distance the switch operator travels between the closed and opened positions of the switch). If an on-off system is operated at high speed and a narrow dead band, the system tends to overshoot the control point, causing cyclical operation. It is necessary, therefore, to operate the system at a high speed and wide dead band which allows too much error or at a low speed and a narrow dead band which results in a slow response. The proportional system of the prior art relies upon a very complicated parallelogram linkage. The prior art system also does not allow the horizontal orientation of the blade circle and drawbar assembly to be predetermined.